1. Field of the Invention
The present invention relates to an optical information readout apparatus having an optical pickup for an optical information record medium such as an optical disc, a magneto-optical disc or the like. More particularly, it relates to a laser resonator type optical information readout apparatus capable of reading out information recorded in a record medium by utilizing the surface of the record medium as a part of the laser resonator and by detecting changes in a laser resonance.
2. Discussion of Background
FIG. 5 shows the basic construction of an optical pickup for an optical disc or a magneto-optical disc. Laser beams excited from a semiconductor laser 1 are rendered to be parallel beams by a collimator lens 2. The parallel beams are passed through a polarizing beam splitter 3 with a predetermined plane of polarization. Then, the beams are passed through a .lambda./4 plate 4 so that a state of polarization is changed, and are converged by a focusing lens 5 to be irradiated to a record medium 7. The beams are reflected at the surface of the record medium 7 and are again passed through the .lambda./4 plate 4 where the plane of polarization is changed. Namely, the plane of polarization is 90.degree. different from that of the original beams. The beams passed through the .lambda./4 plate 4 are reflected at the polarizing beam splitter 3 to be detected by a detector 6.
The conventional optical pickup having the construction described above has problems that the weight is heavy and the manufacturing cost is high because it needs a large number of structural elements and is therefore bulky. Further, because of the above-mentioned reason, adjusting operations such as aligning an optical axis and aligning positions for a number of structural elements are complicated. Furthermore, an additional optical device is needed so that light to be irradiated to the record medium 7 is rendered to be linearly polarized light when a change of rotation of the plane of polarization in a magneto-optical disc has to be read.